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For the current study, researchers culled previously published data that contained information about the distances between the Milky Way, Andromeda and other close-by galaxies—including those that weren’t satellites—that reside in and right outside an area referred to as the Local Group.

Galaxies in the Local Group are bound together by their collective gravity. As a result, while most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity.

For the first time, researchers were able to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda.

90% dark matter

“Historically, estimations of the Milky Way’s mass have been all over the map,” says Matthew Walker, assistant professor of physics at Carnegie Mellon University.

“By studying two massive galaxies that are close to each other and the galaxies that surround them, we can take what we know about gravity and pair that with what we know about expansion to get an accurate account of the mass contained in each galaxy. This is the first time we’ve been able to measure these two things simultaneously.”

By studying both the galaxies in and immediately outside the Local Group, Walker was able to pinpoint the group’s center. The researchers then calculated the mass of both the ordinary, visible matter and the invisible dark matter throughout both galaxies based on each galaxy’s present location within the Local Group.

Andromeda had twice as much mass as the Milky Way, and in both galaxies 90 percent of the mass was made up of dark matter.

The UK’s Science and Technology Facilities Council supported the research, which was led by Jorge Peñarrubia of the University of Edinburgh’s School of Physics and Astronomy.

Coauthors include Yin-Zhe Ma of the University of British Columbia and Alan McConnachie of the NRC Herzberg Institute of Astrophysics.